The present invention relates to a digital transmission network and in particular to a digital transmission network which is suitable for transmission of video, audio signals, etc. to monitor a plurality of monitor sites dispersed in a wide area through video and audio signals by devices in monitor centers which are provided in one or more locations, in which terminals each including a monitor device are provided in the plurality of monitor sites.
System configurations of conventional video transmission networks are shown in FIGS. 7 and 8. FIG. 7 shows a system configuration in which a plurality of cameras 71 and a plurality of monitors 72 are all disposed in a star configuration and are connected to a central unit 73 so that all connection and switching between the cameras and the monitors is carried out by a matrix switch in a central unit.
FIG. 8 shows a so-called LAN (local area network) configuration in which a plurality of cameras 81 and monitors 82 are all connected through terminals 83 and a communication control unit 84 in the form of loop so that connection and switching between the cameras and monitors is carried out.
In the system of FIG. 7, the monitor 72 and the central unit 73 are usually disposed in the same position although the cameras 71 are dispersed. In a large scale monitor system, the distances between the cameras and between the cameras and the central unit are often several kilometers to several tens of kilometers.
For further information on the technology related with the present application, reference may be made to U.S. Pat. Nos. 4,937,823 and 5,101,405.
If the monitor area is wide and the number of monitor sites is large in the system of FIG. 7, the amount of the transmission line would be increased, and the matrix switch in the central unit would have an increased capacity and become complicated.
In the system of FIG. 8, a so-called Ethernet, token ring, or other format in which a plurality of data are randomly superposed on a looped transmission line is often adopted. Since the rate of the data of the monitor cameras is remarkably larger in comparison with that of usual personal computer and the period of its connection occupation time is extended, a waiting time for connection may occur on congestion of data and the system may often be inappropriate for emergency monitor. In particular, in case where a number of video data are concentrated on occurrence of accident, the connection will be impossible.